Amusing setter's latest splits party fifty-fifty (5)
I believe the answer is:
droll
'amusing' is the definition.
(I've seen this in another clue)
'setter's latest splits party fifty-fifty' is the wordplay.
'latest' indicates one should take the final letters.
'splits' indicates putting letters inside (inserted letters split open the word).
'party' becomes 'do' (do is a kind of party).
'fifty-fifty' becomes 'LL' (L is 50 in Roman numerals).
The final letter of 'setter' is 'r'.
'r' going inside 'do' is 'dro'.
'dro'+'ll'='DROLL'
(Other definitions for droll that I've seen before include "Humorous or whimsical" , "Strangely amusing" , "Amusing in offbeat way" , "Dry" , "Whimsically comical" .)